The majority of uranium recovery processes target the enrichment of uranium-bearing ore, whereby the uranium concentration of the solution into which it is leached is generally in the region of a few grams per liter. The leach solution is usually sulphate-based, although alkaline leaching has also been used. Due to the growing demand for uranium worldwide, to an increasing extent ores in which uranium is not the main metal but only a secondary product appearing in small concentrations have had to be used as the raw material source. These kinds of raw material sources are in particular solutions occurring in the phosphoric acid, copper and rare earth metal industries as well as the effluents from oil shale mines. The concentrations in these solutions may even be less than 20 ppm. In such cases the grounds for uranium recovery may also be environmental requirements. The recovery of uranium from such a solution is technically and economically more demanding, because the costs of the recovery process must not become too great in relation to the value of the product generated.
Solvent extraction was first adapted for the recovery of uranium on a large scale in the mid-1950s using an extractant including the ingredient bis(2-ethylhexyl) phosphate (CAS No. 298-07-7). This reagent is often referred to in the literature by the name di(2-ethylhexyl) phosphoric acid or by the abbreviation D2EHPA. However, fairly quickly the use of extractants based on tertiary amines became more common. The reason for the popularity of amines was particularly the fact that they were found to have better selectivity with regard to certain impurities such as ferric iron and rare earth metals. Nowadays most new plants use tertiary amines. On the other hand, D2EHPA has numerous other applications e.g. in the hydrometallurgy of zinc, cobalt and nickel. The advantages of D2EHPA over tertiary amines are its significantly lower price, the fact that it is a more powerful extractant and that it is safer for the environment.
In uranium extraction plants known to use the D2EHPA extraction reagent, the recovery of uranium generally occurs with an aqueous solution of sodium or ammonium carbonate. D2EHPA saponifies in stripping and a third liquid phase is formed between the organic solvent and the aqueous phase, which can be prevented with a suitable non-ionic surface-active substance i.e. a modifying agent. Long-chain alcohols among other things have been used in uranium processes, as have alkyl phosphates, alkyl phosphonates and alkyl phosphine oxides. U.S. Pat. No. 2,859,094 describes uranium extraction that takes place from an aqueous solution of sulphuric acid, in which the sulphuric acid concentration is 1.5 M and that of uranium around 1 g/L. As stated in the patent, the modifying agents listed above have a beneficial synergistic effect on the distribution ratio of uranium. One preferred organophosphorus compound mentioned is tributyl phosphine oxide.
A uranium-containing solution often also includes other metals such as iron, aluminium, vanadium, molybdenum, manganese, nickel and rare earth metals. The extraction process must be constructed so that as little of the undesirable substances as possible is extracted along with the uranium.
The article by El-Nadi et al: “Sulphide precipitation of Iron and its Effect on the Extraction of Uranium from Phosphoric Acid Medium”, The Japan Society of Nuclear and Radiochemical Sciences, published on the Internet on Jun. 23, 2004, describes phosphoric acid-based leaching in which iron is precipitated from a uranium-bearing aqueous solution by means of sodium sulphide before the solution is routed to extraction. The phosphoric acid concentration of the aqueous solution is 5M, i.e. around 490 g/l. The extractant is D2EHPA and the modifying agent a straight chain trioctyl phosphine oxide, TOPO, which is produced for instance under the commercial trade name CYANEX 921, and has a melting point of 47-52° C. The precipitation of trivalent iron from the aqueous solution has been performed by adding solid sodium sulphide to the solution while simultaneously mixing, so that the iron is precipitated as iron sulphide and, in addition, elemental sulphur is also formed. After this the solution is thickened and filtered to remove the solids and only then is the solution routed to extraction.
Two cases of uranium extraction carried out in Colorado are described on pages 510-511 of the book by Ritcey, G. M.: “Solvent Extraction” vol. 2, Ottawa, Canada, 2006. Climax Uranium has used a process in which uranium is recovered from a solution containing sulphuric acid by extraction, in which the extractant is D2EHPA, the modifying agent tributyl phosphate and the solvent kerosene. Before extraction the iron in the solution is reduced to divalent, so that it is not extracted with the uranium. Uranium was recovered with soda ash, after which the solution was acidified with sulphuric acid and the uranium precipitated by means of ammonia. Cotter Corporation has used a process in which the extractant is also D2EHPA and the modifying agent isodecanol. In the extraction, uranium is separated from cobalt and nickel in four extraction steps. Stripping is performed in three steps by means of sodium carbonate.